This invention relates to electronic instruments for detecting the presence of trace amounts of gaseous impurities in the ambiant atmosphere, it is directed particularly to a portable electronic device for the detection of Freon leaks or the leakage of other gaseous hallides into the surrounding atmosphere in the maintenance and repair of air conditioning and refrigeration systems, for example.
Various types of portable electric instruments utilizing high voltage corona discharge gaps as sensing means in the detection of minute concentrations of foreign gasses in air such as one of the gaseous hallides, have heretofore been devised. Such detectors, which rely on the changes in the sensing gap conductance as being a function of concentration in the ambiant atmosphere of the gaseous impurities to be measured, have found wide application in the detection of refrigerant leaks, such as leakage of Freon in the maintenance and repair of commercial, residential and automotive air-conditioning systems. Typically, the magnitude of the voltage applied to a sensing tip comprising the corona discharge gap is manually adjusted until a corona is stricken and a given current flows in the discharge. This current is utilized to energize an audible alarm circuit producing "clicks" or monotonic "beeping" sounds in a loudspeaker at a rate inversely proportional to the current--the greater the current the longer the time interval between "clicks" or "beeps". The component values of the alarm circuit are selected to produce a predetermined "clicking" rate presumed to be readily recognizable when the optimum current flows through the sensing tip corona. In use of such gas detectors, the operator adjusts the high voltage until the right "clicking" or "beeping" rate is obtained. Since the detecting signal of gaseous concentration in the air being measured is an audible "clicking" or "beeping" signal the repetition rate of which increases with increased concentrations of the foreign gas, the signal repetition frequency is not a consistent indicator of a given concentration. Thus, because of perceptual variations, what seems like a optimum "clicking" or "beeping" rate to one operator ordinarily does not to another operator, optimum sensing tip current flow adjustment is rarely achieved, with consequent decrease in instrument sensitivity. Additionally, the operator is distracted because of the nearly continuous need to adjust the high voltage being applied to the sensing tip discharge gap in an attempt to maintain optimum current flow or sensitivity adjustment in the face of a changing gaseous mixture environment. These deficiencies necessitate inordinate attention to the sensing readjustment of the instrument, especially in the detection of small leakage rates of 0.1 oz. per year, for example.